The present invention relates to ceramic superconductors and composite materials incorporating ceramic or other brittle superconductor materials. The invention relates to many superconductor materials including but not limited to perovskite/cuprate superconductors, A15 compounds, Laves phase superconductors and Chevrel phase superconductors.
The discovery of high critical temperature (Tc) superconducting ceramics (HTS ceramics) has inspired an enormous interest in their application. Conventional niobium alloy superconductors such as NbTi must be cooled to below 10 K to achieve useful superconductivity. HTS superconductors, on the other hand, can have Tcs over 100 K. Due to the great expense of cryogenic refrigeration, the HTS ceramics could find much wider application in industrial and laboratory devices. Of particular interest are materials which have Tc above 77 K, because this is the temperature of liquid nitrogen, a common and relatively inexpensive refrigerant. HTS ceramics have not been used in many potential applications because they suffer from a number of shortcomings. The most severe problems with the HTS ceramics are as follows:
1) HTS ceramics are brittle. They are not flexible and thus cannot be made into wires or other useful shapes. Cracks and boundaries between adjacent crystals severely limit supercurrent flow.
2) HTS ceramics are highly anisotropic. Supercurrents preferentially flow in certain directions with respect to the crystal lattice, reducing the maximum current density in randomly oriented multicrystalline pieces.
3) HTS ceramics are strong oxidizing agents. Most metals, such as copper, lead, tin, aluminum, indium and niobium, are oxidized by contact with the ceramic superconductors. Insulating oxide layers impede supercurrent flow. Only noble metals such as gold, silver, palladium and their alloys are not oxidized by the HTS ceramics.
A less severe undesirable feature of the HTS ceramics is that they can lose their superconducting properties under certain circumstances. The superconducting structure inside the HTS ceramics has an abundance of oxygen atoms which are necessary for superconductivity. Heating, grinding, etching, or prolonged exposure to ambient atmosphere or vacuum may liberate the oxygen and destroy superconductivity. Both the oxygen content and the superconductivity can be restored by annealing the HTS ceramic in an atmosphere of oxygen.
It would be an advance in the art of applied superconductivity to provide a superconducting wire employing HTS ceramics that is ductile, has a high Tc, and has a high critical current density (Jc). Such a wire must overcome the problems with the HTS ceramics. Prior art HTS ceramic wires made of a combination of HTS ceramic particles in a silver matrix generally have poor superconducting properties such as low Jc. Also, bending the prior art wires tends to greatly reduce the Jc. This is highly undesirable.
There are other superconductor materials which have some of the same disadvantages as the HTS cuprate materials. For example, the A15 family of superconductors such as Nb3Sn are also brittle materials (although they are not anisotropic and relatively nonreactive). Their poor mechanical properties have precluded their use in many applications requiring ductility such as wires. This is unfortunate because they generally have good superconducting properties such as relatively high Tcs, high critical magnetic fields, and high critical current densities.
Other examples of brittle, nonductile superconductors include materials possessing the NaCl crystal structure (the AB family), Laves phase materials, and Chevrel phase materials. These materials may have superior superconducting properties, but are unusable in many applications (e.g., conducting wires) because they are brittle. It would be an advance in the art of applied superconductivity to provide flexible wires made from brittle superconductor materials.
U.S. Pat. No. 5,091,362 to Ferrando discloses a method for forming a silver coating on HTS ceramic particles. U.S. Pat. No. 4,971,944 to Charles et al. teaches a method for electroless deposition of gold onto HTS ceramic particles.
U.S. Pat. No. 5,041,416 to Wilson describes a superconducting composite material. Powders of HTS ceramic and normal metal are mixed and the mixture is subjected to heat and high pressure. The composite materials of Wilson have a relatively low Jc due to reactivity between the HTS ceramics and the metal matrix. The wires also have a low Jc if silver is used as the normal metal.
U.S. Pat. No. 5,202,307 to Hayashi describes a superconducting composite material having HTS ceramic particles in a metal matrix. The composite materials of Hayashi have a relatively low Jc due to reactivity between the HTS ceramic particles and the metal matrix and/or due to poor superconducting properties of the metal matrix materials.
U.S. Pat. No. 5,194,420 to Akihama describes a composite cuprate superconductor/metal superconducting material consisting of HTS ceramic particles dispersed in a matrix of silver. The composite materials of Akihama will also have a relatively low critical current density due to the choice of silver as the metal matrix material.
U.S. Pat. No. 5,081,072 to Hosokawa et al. describes a method of preparing a HTS superconducting ceramic powder and forming the powder into a superconducting material. A low Jc is also a problem with the materials of Hosokawa.
U.S. Pat. No. 5,547,924 to Ito et al. describes a superconducting ceramic composite material having HTS ceramic particles in a noble metal matrix. The composite materials have relatively poor superconducting properties due to the poor superconducting properties of the metal matrix materials used.
U.S. Pat. No. 5,132,278 to Stevens et al. describes a cuprate superconductor wire having continuous filaments of HTS ceramic surrounded by a metal matrix. A noble metal chemically protects the HTS ceramic. The wires of Stevens et al. are characterized in that they do not conduct current between wires, and do not rely on the superconducting proximity effect.
There exists a need for a HTS ceramic superconducting material that is ductile and has a high Jc that is not reduced by bending. Also, there exists a need for ductile superconducting materials made from brittle superconductors that have a high Jc high Tc, and high ductility.
These objects and advantages are provided by a superconducting composite material having superconductor particles made of superconductor material disposed in a metal matrix material. The superconductor particles have dimensions larger than the superconducting coherence length of the superconductor material. The metal matrix material has an electron-boson coupling (typically electron-phonon coupling) coefficient greater than 0.2. The superconductor particles are coupled by a continuous superconducting path due to the proximity effect when the composite material is cooled below the critical temperature of the superconductor particles.
The superconductor particles preferably have dimensions in the range of about 2 nanometers to 10 microns, more preferably in the range of 10-500 nanometers. Larger particle sizes may also be used. The superconductor particles can be made of various superconductors including A15 compounds, AB family superconductors, Laves phase superconductors, Chevrel phase superconductors, and HTS ceramic superconductors. In the present specification, an HTS superconductor is a superconductor with a Tc greater than 30 Kelvin. The cuprate superconductors are well known examples of HTS superconductors.
The metal matrix material preferably has a high electron-phonon coupling coefficient, xcex. All else being equal, the xcex of the metal matrix material is preferably greater than 0.2, more preferably greater than 0.5, yet more preferably greater than 1.0, and most preferably greater than 1.5 (i.e., higher values are more preferred). The metal matrix material can be made, for example, of niobium, indium, NbTi alloy, tin, lead, lead/bismuth alloys, mercury, tantalum, titanium, vanadium, titanium/bismuth alloys, lead/titanium alloys and alloys thereof. All these materials have xcex values greater than 0.5.
The metal matrix material can be pure elemental metal, or an alloy. The metal matrix material can comprise a mixture of different metals, in which case the metal matrix material can even include materials having low xcex values (e.g., metals with xcex values less than 0.2 such as silver). This allows control over certain properties of the composite material (e.g., n-value, Tc, and Jc).
In another embodiment of the present invention, the superconductor particles have a metal coating separating the superconductor particles from the metal matrix material. The coating is preferably thick enough to prevent chemical reactions between the superconductor particles and metal matrix material. The coating is highly preferred in embodiments where the superconductor particles and the metal matrix material are chemically incompatible. The coating is chemically compatible with the superconducting particles and the metal matrix material. In the present description, xe2x80x98chemically compatiblexe2x80x99 means that the metal coating does not destroy the superconducting properties of the superconductor particles and does not destroy the superconducting properties of the metal matrix material. Also, being xe2x80x9cchemically compatiblexe2x80x9d means that the metal coating does not form an insulating layer at the interfaces. A small amount of degradation is within the scope of the present invention.
The superconductor particles can be made of HTS ceramics. Most HTS ceramic particles require a metal coating because most HTS ceramics are chemically reactive with high-xcex metals. The coating preferably is made of a noble metal (e.g., silver, gold, palladium, or an alloy thereof) that is not oxidized by contact with the HTS ceramic.
The metal coating is preferably as thin as possible. The metal coating is preferably thinner than the electron mean free path of the metal coating material at the Tc of the superconductor particles. More preferably, the metal coating is thinner than the proximity effect decay length of the metal coating at the Tc of the superconductor particles. The coating is preferably in the range of 2-3000 nanometers thick, more preferably in the range of 2-50 nanometers thick. Alternatively, the metal coating is preferably thinner than the electron mean free path of the metal coating material at 4.2 Kelvin. Also, the metal coating is preferably thinner than the proximity effect decay length at 4.2 Kelvin.